1. Field of the Invention
The present invention relates to a printer that uses a thermal printing technique, and relates more particularly to a thermal printer having a print head pressure mechanism for pressing a thermal print head against a platen roller.
2. Description of the Related Art
A print head pressure mechanism 100 according to the related art is shown in FIGS. 8(a) and 8(b). This print head pressure mechanism 100 has a thermal print head 101, platen roller 102, and compression springs 109, comprised such that the compression springs 109 push the thermal print head 101 against the platen roller 102 for printing.
The heat elements of the thermal print head 101 are disposed on a ceramic substrate having a driver IC mounted thereon. The ceramic substrate is supported on a head support base 103, which functions as a heat radiator. The head support base 103 is basically rectangular with support shafts 104, 105 disposed coaxially to each other in the longitudinal direction of the head support base 103. The thermal print head 101 is supported by these support shafts 104, 105 so that it can pivot relative to the printer body 106.
An axle 108 passes longitudinally through platen roller 102. The axle 108 is rotationally supported by printer body 106 with the axle 108 parallel to the longitudinal axis of the support shafts 104, 105 of head support base 103.
A plurality of compression springs 109 push against the back (that is, the side opposite the side supporting the ceramic substrate) of thermal print head 101, urging the thermal print head 101 in the direction of the platen roller so that pressure will be evenly applied along the contact line between thermal print head 101 and platen roller 102.
A problem with a print head pressure mechanism 100 according to the related art as described above is that the relationship between the longitudinal axis of the head support base 103 and the longitudinal axis of the platen roller 102 deviates from the expected parallel relationship due, for example, to variations in the manufacturing precision of various parts. This means that the pressure between the thermal print head 101 and platen roller 102 is not actually uniform. A uniform print density can therefore not be achieved.
It is an object of the present invention to overcome the aforementioned problem of the prior art, and to provide a thermal printer having a print head pressure mechanism that can maintain uniform pressure between the thermal print head and the platen roller without being affected by variations in component precision, and which can therefore print with uniform print density.
To achieve this and other objects, a print head pressure mechanism according to the present invention has a platen roller with a platen shaft extending longitudinally therethrough so that the platen rotates around the platen shaft; a print head support having a thermal print head of a length able to print using a thermal printing method to a recording medium held between the print head support and the platen roller, and a support shaft parallel to the thermal print head, the print head support being movable along a specific path pivoting on the support shaft at one end; and a pressure member disposed at a particular position on the side opposite the thermal print head side of the print head support.
Because one end of the print head support is held fixed at one end of the support shaft while the other end of the support shaft is moved to alignment with the platen roller, the thermal print head of the print head support can contact the platen roller uniformly regardless of the positioning precision of the platen roller.
If the pressure member is positioned so that pressure is applied evenly to the contact parts of the platen roller and the thermal print head of the print head support, the thermal print head can be held uniformly against the recording medium. Printing with uniform print density is therefore possible regardless of variations in parts precision.
Further, the print head pressure mechanism includes a positioning member for positioning the one end of the support shaft of the print head support, and a guide member for guiding along a specific path the other end of the support shaft of the print head support. The print head support is reliably guided through a specific path to the platen roller.
Further, the pressure member is disposed with a pressure working point on a line offset by a distance to the support shaft side of the print head support from the contact line between the thermal print head and platen roller, and distributed equally with respect to a specific reference point on said line. This configuration assures that uniform pressure is applied to the contact line between the thermal print head and platen roller.
Further, the pressure member is disposed offset by a distance from the position determined relative to the specific reference point.
The shift in the working point of the load from the pressure member during actual printing can be obtained by computer analysis using various external factors contributing to the shift. By offsetting the positions of the pressure members the distance determined by this computer analysis from the position of static balance, which is achieved by positioning the pressure member as described above according to the present invention, a so-called dynamic balance can be achieved during printing.
A printer according to the present invention has a printer body capable of holding a recording medium; and a positioning support member for disposing a print head pressure mechanism according to the present invention inside the printer body for printing to the recording medium, the positioning support member being disposed to the platen roller drive side of the printer body.
Preferably in this case, the printer has a printer cover that opens and closes to the printer body. The platen roller of the print head pressure mechanism is disposed on the printer cover so that the platen roller approaches and separates from the thermal print head in conjunction with printer cover opening and closing.
It is therefore possible for the present invention to provide a printer that can print to a recording medium with uniform print density regardless of variations in parts precision.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.